1. Field of the Invention
This invention generally relates to information recording and reproduction apparatus and, more especially, to apparatus for finding the center of an information-bearing track without the use of auxiliary tracking heads or servo tracks.
2. Description Relative to the Prior Art
In order to recover information recorded on a circular track on a rotary storage medium, it is important to center a playback head on the part of the track producing the greatest signal strength. This is ordinarily the center of the track, where the signal envelope corresponds to maximum signal amplitude. Finding the center based on amplitude measurement is difficult for a number of reasons: interference between tracks produces anomalous amplitude readings, different signal levels from medium to medium and player to player necessitate dealing with a variety of signal levels, and the signal drop from outer to inner tracks on the same medium complicates the interpretation of signal levels.
U.S. Pat. 4,204,234 describes a track-following servo in which a playback head follows a path defined by the maximum amplitude data signals. The crest or maximum amplitude signal is found by the analysis of a regular sequence of digitized amplitude samples. The track center is found by observing that the present sample (third sample, in time) is less than the previous sample (second sample), which in turn was greater than the one prior to it (first sample). A position error signal is calculated from these amplitude samples and input to the servo to direct the head backwards toward the track center. In a further aspect of this technique, a fourth amplitude sample strengthens the criteria for crest detection if it is less than the third sample. This track-following technique is intended for following an existing, known track where a signal crest is assured. Its reliance on the amplitude value of adjacent samples, however, is susceptible to anomalous signal levels characteristic of unknown tracks and unknown media, a situation such as encountered when a medium is first put into a player.
It is known to base a tracking decision upon a threshold related to the expected signal level. U.S. Pat. 4,689,700 describes a thresholding technique in which track center is found by determining the positions where the envelope signal strength passes a fixed threshold on both its upward and downward slopes. Track center is then calculated to be midway between the two points. Since average signal strength drops from outer to inner tracks and a fixed threshold could miss a track, the '700 patent allows for varying the threshold level depending on the position of the track. This technique, however, is based on implicit foreknowledge of the expected signal. Noisy signal levels or unusual signal levels, which are dependent on the vagaries of the recording environment, will still lead to erroneous estimation of track centers.